Proteins that bind to specific sites on the surface of human animal erythrocytes and/or to sites on other tissues have been variously called agglutinins, lectins, antibody like substances or receptor specific proteins (RSPs). We prefer the more general term RSP which has been defined to mean proteins or glycoproteins that recognize and bind to specific receptors but which have no known biological function as do hormones, drugs and neurotransmitters. Gold and Balding Receptor Specific Proteins, American Elsevier Publishing New York (1975). The value of RSPs for such uses as blood group determination, bacterial typing, histological and cytochemical staining, cell separation and mitogenic stimulation of lymphocytes has become readily apparent. Lis and Sharon Ann. Rev. Biochem. 55: 35-67 (1986).
The first blood group specific RSPs were obtained from extracts of certain plants. Race and Sanger, Blood Groups in Man Blackwell Scientific Publications 6th ed Oxford pp. (1975). These RSPs were specific for the A.sub.1 antigen of the ABO blood group system and could be used to distinguish A.sub.1 from A.sub.2 cells. Presently, a variety of RSPs found in extracts of plant or animal tissues are known to have blood group specificity. Race and Sanger; Gold and Balding (1975). Examples include RSPs with specificity for the A.sub.1, A, D, M, N, T, Sd and Cad serological groups. Although RSP blood typing is limited by the small number of blood groups recognized, RSPs have an advantage over antibody preparations because they are more specific and do not cross-react with closely related antigens.
Adherence of bacteria to animal erythrocytes was first reported in 1908. Guyout, Zentialblatt fur Bakteriologie, Parasitenkunde, Infektionskrankhecten und Hygiene 47: 640-653 (1908). Subsequently, it was demonstrated that organized fibrous appendages, on the surface of bacteria, called fimbriae or pili, mediated hemagglutination. Duguid J. Path. Bact. 70: 335-348 (1955). The adherence organelles have an important role as colonization factors in the early stages of bacterial infection. Initially, the binding specificity of these RSPs was defined by the amount of D-mannose inhibition of adherence. The bacteria were classified as mannose sensitive (MS) or mannose resistant (MR). The MS RSPs are presumed to recognize a mannose containing receptor, are called "Type 1" fimbriae, and have been identified on many members of the family Enterobacteriaceae. On the other hand, MR RSPs are divided into groups based on their antigenic cross reactivity, subunit size and binding specificity for tissues and erythrocytes of various animal species. MR RSPs include K88, K99, 987P, F41, CFA/I, CFA/II, P, M, S and G. Of this group, only the P and M RSPs recognize specific blood group antigens. Bacteria that express P RSP will agglutinate P erythrocytes but not p erythrocytes while bacteria that express M RSP will agglutinate MM and MN but not NN erythrocytes. Although plant RSPs specific for MM and MN cells are easily obtained from several species in the genus Iberis, other RSPs specific to blood group antigens have been rarely discovered.